Circuits contained in integrated circuit devices (ICs) or printed circuit boards (PCBs) are often composed of multiple layers of signal lines separated by insulating non-conductive layers. Signals are transmitted between the signal lines of different layers with vias. Vias are holes between non-conductive layers that have been made conductive by electroplating, use of annular rings, or small rivets.
The signal lines in the circuit are typically of a width and spaced with respect to nearby ground and power lines or planes such that they have a standard characteristic impedance, such as 50 ohms. This is done to reduce signal fluctuation and distortion caused by impedance mismatches. Matching of impedance is important because a mismatch will result in a signal being reflected at the point of the mismatch. The reflected signal results in noise and distorts the signal propagated forward. The drawbacks of impedance mismatch are particularly acute in high speed circuitry where the clock cycle may dictate that the signal propagated in a circuit remains at a stable value for a short period of time relative to the gradual rise and fall generated as a transistor switches on or off. Noise and distortion generated by impedance mismatches may be recognized as a switching transition.
Because vias for differential signals are typically of a lower impedance than the signal lines connected to them, the impedance mismatch may require circuits to be operated at a frequency far lower than desired. One or more embodiments of the present invention may address one or more of the above issues.